The invention relates to neutral color detection. It finds particular application in conjunction with neutral pixel detection in image data using color space feature vectors wherein one color space coordinate represents lightness and will be described with particular reference thereto. However, it is to be appreciated that the invention is also amenable to other applications.
Neutral colors include black, white, gray, and colors containing a predetermined amount of gray. In a three-dimensional color space where one of the color space coordinates represents lightness (i.e., luminance), neutral colors are grouped on or near the lightness axis of the color space. One such color space is commonly known as the L*a*b* color space.
In the L*a*b* color space, values L*, a*, and b* are plotted at right angles to one another to form a three dimensional coordinate system. Equal distances in the color space represent equal color differences. Value L* represents a position with respect to a lightness (i.e., L*) axis; value a* represents a position with respect to a redness/greenness (i.e., a*) axis; and value b* represents a position with respect to a yellowness/blueness (i.e., b*) axis. The L*a*b* color space is also referred to as CIELAB, CIE L*a*b*, or CIE Lab. CIE (Commission International de L'Eclairage) is an international commission on illumination that is the main international organization concerned with color and color measurement.
In color image processing, reproduction, recording and the like, a page of image data is read and typically a three-color, cyan-magenta-yellow (CMY), system is employed for rendering. Optionally, a fourth color, black (K), is included. Generally, combinations of the colors CMY are applied in relative proportions to produce the color portions of the image. Additionally, the CMY colors can be applied to produce a resultant black. However, the resultant black achieved by combining the CMY colors is not as high quality as that achieved with the true black (K). Chromatic noise, misregistration, and the like can degrade the quality of black images generated by a CMY resultant black application. Moreover, multiple CMY passes are relatively more time consuming compared to a K pass, and they increase the depletion of CMY inks and/or toners. Generally, these and other inherent constraints, limitations, and/or reproduction demands associated with color processing and/or printing make it desirable to limit CMY utilization where possible.
In the past, there have been developed techniques for determining if input pages are neutral in color (i.e. black and white, or monochromatic) to thereby reduce CMY utilization where it was not necessary. For example, in U.S. Pat. No. 5,287,204 to Koizumi et al. and U.S. Pat. No. 5,282,026 to Nagata, both incorporated herein by reference, such techniques are disclosed. However, both references achieve limited results. That is to say, the references are directed to detecting at the page level and determining if an input page is neutral in color. Generally with these techniques, each page as a whole is classified as color or neutral, which results in either the whole page undergoing color processing or none of the page undergoing color processing. As such, these approaches fail to address, for example, chromatic noise in a neutral region of an otherwise color page. Moreover, no provisions are made for high quality neutral detail, such as text, that may be located in a color page or region thereof.
A more recent method of neutral color detection is disclosed in U.S. Pat. No. 6,249,592 to Fan et al., incorporated herein by reference. This method provides neutral color detection of a whole page or one or more regions in a page. This method includes reading an input page. The contents of the page are detected and it is determined if the page is neutral in color. If the page is not neutral in color, the page is then divided into a number of regions. Each region is then detected, and a determination is made if individual regions are neutral in color. Next, details located outside regions determined to be neutral in color are detected and it is determined if the details are neutral in color. This multi-resolution technique permits the full page, an individual region, and/or particular details to be processed without the constraints, limitations, and reproduction demands associated with their color counterparts. However, a drawback of this method is that it requires the computation of images at multiple resolutions in order to judge whether a region is neutral or not. This can be time consuming in applications.
Color image paths in copiers, scanners, and printers must maintain performance at a certain page per minute (ppm) level (for example, but not limited to, 20–100 ppm). Image processing within the color image path requires substantial storage space (e.g., 90 Mbytes per page). If an entire document image is deemed neutral, color image processing can be bypassed, saving time, memory, and color ink or toner. If a region of a document image is deemed neutral, the image can be processed using a halftoning algorithm selected to increase image quality. If a pixel in a document image is deemed neutral, the pixel can be printed with black ink or toner.